The present invention relates generally to carbon nanofibers and nanotubes, and more specifically to a post-manufacture processing method for debulking, dispersing, purifying, surface treating, pre-impregnating, and micronizing filamentary nanocarbon.
The term filamentary nanocarbon is meant to encompass carbon nanotubes, both single and multiwall, as well as carbon nanofibers. Filamentary nanocarbon is a recently developed material having great commercial utility and promise for providing structural support or reinforcement, electrical and thermal conductivity and for use in electronic devices including transistors and solar cells. Other uses of filamentary nanocarbon include composites, filled polymers, electron emitters and flat panel displays.
The filamentary nanocarbon in its as-produced state is often very bulky, with the product occupying a large volume to mass ratio. It is frequently desirable to reduce this bulk for several reasons including ease of handling, shipping and packaging as well as reduction of the propensity of the tiny fibers to become airborne.
Typically, filamentary nanocarbon is tightly tangled and very often must be disentangled to satisfactorily impart the desired properties to the materials and devices incorporating it. For example, a tangled mass of filamentary nanocarbon, when combined with an insulating polymer, will not conduct electricity as well as the same amount of filamentary nanocarbon that is well dispersed. Additionally dispersion of single wall nanotube ropes is necessary to separate them by type (i.e., semiconducting vs. metallic). The prior art processes which are used to disperse or disentangle the filamentary nanocarbon disadvantageously break the fibers into shorter lengths. This can have the effect of reducing the electrical and thermal conductivities and the structural reinforcement efficacy of the product produced.
In addition to the fiber tangling problem described above, most filamentary nanocarbon materials produced contain metal catalyst. They also may contain polyaromatic hydrocarbons, soot, and non-filamentary carbon particles all of which are desirably removed in order to leave only the nanofibers for end product use.
The surface of an as-produced carbon nanotube or nanofiber may not be compatible with a given polymer, i.e., it may not wet or adhere well. This leads to difficulty in dispersing the product in a matrix (e.g., a thermoplastic or thermoset resin), as well as to poor overall mechanical properties in filled polymers or composites due to facile matrix/filler (reinforcement) debonding. The currently established methods for filamentary nanocarbon surface treatments to render them compatible for dispersion into a polymer include acid treatments, partial gas oxidation and electrochemical reaction. Acid and electrochemical treatments are intrinsically liquid processes, either aqueous or non-aqueous.
Pre-impregnation (or prepregging) is a technique of coating or infiltrating a fiber bundle or mat with a polymer before making the final article. For discontinuous materials (e.g., fillers) this can be thought of as masterbatching or dispersion into a polymer. Once prepregged, the material can be blended with additional material or formed directly into the desired shape or infiltrated into a conventional (non-nano) fiber architecture (e.g., mat, weave cross-ply, tow, etc.). Pre-impregnation is desirable because of the greater control and ease of dispersion and the reduction of aerosolized fiber during manufacturing.
The prior art methods for nanofiber purification, debulking, surface treatment, and dispersion frequently utilize liquids such as water, acids and alcohols which must be removed after treatment. This is a costly and time consuming step because the fibers must be dried before future use, which takes a great deal of time because the fibers have a high relative surface area.
A need exists therefore for an improved method for processing filamentary nanocarbon for purification, debulking, surface treatment, polymer pre-impregnation, and dispersion. Such a method would provide effective nanofiber processing, utilize commonly available apparatuses and equipment, and provide reduced processing costs.